(1) Field of the Invention
The present invention relates to a process for producing a semiconductor device, and more particularly, to a film forming method by spin coating technique.
(2) Description of the Related Art
In the manufacturing process of semiconductor devices, spin coating method has been widely used in order to form a film of an organic solvent on a substrate. That is, the organic solvent is supplied dropwise on a surface of the substrate as it is rotated. Meanwhile, for semiconductor devices, a stress is generated between a mold resin and a passivation film, in dependence upon variation of temperature. The stress becomes large as the dimension of semiconductor chip is increased. In order to relieve this kind of stress, a polyimide layer may, in some cases, be provided between the mold resin and the passivation film. The above mentioned spin coating method would be used to form such a polyimide layer, for instance.
In order for the polyimide layer to function as a buffer layer relieving the stress generated under the influence of temperature changes, the polyimide layer must be enough thick. To obtain such a polyimide layer with a sufficient thickness, it is necessary to use a polyimide precursor solution having a high viscosity. When the poIyimide precursor solution with a high viscosity is coated to a semiconductor substrate as it is rotated, the polyimide precursor solution cannot be thrown away and removed from the periphery of the semiconductor substrate. As a result, the polyimide precursor solution adheres to the backside of the substrate. The adhesion of the polyimide precursor solution of a high viscosity onto the backside of the substrate may induce generation of dust.
In order to prevent the adhesion, a solvent (to be referred to as a backside-rinsing liquid) is discharged so as to dissolve and remove the polyimide precursor liquid adhered on the backside of the substrate.
FIG. 1 is a cross-sectional view of a rotatory applicator, whereas FIG. 2 is a timing chart showing a sequence of coating of a polyimide film in which the polyimide precursor solution is removed using a conventional backside-rinsing liquid. In FIG. 2, axis of abscissa represents processing time, and axis of ordinate represents revolving speed of a semiconductor substrate.
As shown in FIG. 1, the spin coater has a housing 16 comprising a lower cup 13 and an upper cup 12 disposed above the lower cup 13, and the thus constructed housing 16 holds a semiconductor substrate 16 therein. The lower cup 13 is provided with a drain hose 17 for letting liquid run out.
The upper cup 12 is provided at the center thereof with an opening 19, through which an organic solution can be dropped into the housing from a nozzle 11 for supplying the droplet of organic solution. The injecting nozzle 11 is driven by a driver unit 23 to drop a predetermined amount of an organic solution.
Inside the housing, a vacuum chuck 14 for supporting and holding semiconductor substrate 16 is fixed to a vertical rotary shaft 18 with its substrate supporting face horizontal. A semiconductor substrate 16 is set on the supporting face of vacuum chuck 14, and is vacuumed and sucked to be fixed by vacuum chuck 14. As rotary shaft 18 is driven by a driver unit 22, semiconductor substrate 16 thus fixed on vacuum chuck 14 is rotated by driver unit 22 about a vertical axis at the center thereof.
Inside lower cup 13, a nozzle 15 for discharging backside-rinsing liquid is provided under the periphery of semiconductor 16 mounted on vacuum chuck 14, so as to discharge and inject a backside-rinsing liquid on a backside (underside) of semiconductor substrate 16. The discharging nozzle 15 is driven by a driver unit 24 to discharge backside-rinsing liquid in a predetermined amount.
These driver units 22, 23 and 24 are regulated by a controller 21. Now, operation of the spin coater shown in FIG. 1 will be described based on controlling features of controller 21 with reference to a timing chart shown in FIG. 2.
At step 1 shown in FIG. 2 (a period for dropping coating liquid), organic solution-discharge nozzle 11 controlled by driving unit 23 supplies polyimide precursor solution dropwise onto the surface of semiconductor substrate 16 kept at rest.
Next, semiconductor substrate 16 is driven by means of driver unit 22, to raise its revolving speed to 1,000 rpm. At step 2 shown in FIG. 2 (period of rotation at low revolving speed), substrate 16 is rotated at a low revolving speed of 1,000 rpm for five seconds. This rotation at low revolving speed spreads out the dropped polyimide precursor solution over the surface of the substrate 16. This step is called pre-spinning.
Next, the revolving speed of substrate 16 is further raised to 3,000 rpm, and at step 3 (a period of rotation at high revolving speed), substrate 16 is rotated at a high revolving speed of 3,000 rpm for 20 seconds. This rotation is to regulate the thickness of the polyimide film to a desired dimension. The step 3 is called main spinning.
Then, at step 4, with keeping substrate 16 rotating at the same revolving speed as in the main spinning, dimethylacetamide (to be abbreviated as DMAc hereinafter) is injected from nozzle 15 for injecting backside-rinsing liquid by means of driver unit 24. The thus injected DMAc is directed toward the backside of the substrate for thirty seconds, to thereby dissolve and remove the polyimide film adhered onto the backside of the substrate.
At step 5 (a period for drying), the discharge of DMAc is stopped while substrate 16 is kept on rotating for forty seconds at a high revolving speed of 3,000 rpm. During this, the backside of substrate 16 is dried by air flow.
It is known that when a photo-resist film is formed by the spin coating method, ketone is used as a backside-rinsing liquid. On the other hand, in a case where a SOG (spin on glass) film is formed by coating a Si-containing alcohol solution using the spin coating method, alcohols are used as a backside-rinsing liquid. These backside-rinsing liquids are also used, in the same way as the polyimide film is formed, so as to dissolve and remove the film adhered on the backside of substrate.
However, in the case where a film, for example, of polyimide is formed on a substrate by the conventional spin coating method, first, the polyimide film is regulated to a desired thickness (step 3), and thereafter, a solvent must be discharged from the backside-rinsing liquid discharging nozzle 15 while the substrate being rotated at the same revolving speed (step 4). For this reason, the processing takes long time, disadvantageously.
On the other hand, it takes an extremely long time if use of N-methylpyrolidone (to be abbreviated as NMP hereinafter) is made as a solvent for dissolving and removing the polyimide precursor solvent adhered on the backside of substrate 16. For this reason, it is necessary to use a stronger solvent than NMP, such as, for instance DMAc, dimethylformamide (DMF), etc. However, even with DMAc or DMF, it takes about thirty seconds to dissolve and remove the polyimide precursor solution, as shown in FIG. 2. To make matter worse, these solvents exhibit poor volatility, so that the substrate must be spun at a high revolving speed for a long period of time (about forty seconds) in order to dry the solvent (step 5). As a result the processing time for each substrate takes as an extremely long time as about two minutes.
In addition, since the substrate is spun at a high revolving speed for a long period, this poses a problem that the film of polyimide precursor on the substrate surface is made thinner than what is desired. Furthermore, DMAc, DMF, etc. which have strong solvency should be used as the backside-rinsing liquid.
In the case where photo-resist film or SOG film is formed, the solution adhered on the backside of the substrate must be dissolved and removed after the film of a desired thickness has been formed. Accordingly, the processing time for each substrate in these cases also needs a long period of time like the above case of the polyimide film-forming.
On the contrary, if the backside-rinsing liquid is not used, a polyimide precursor solution of a low viscosity must be used. In this case, it is true that, by lowering the viscosity of the polyimide precursor solution, the polyimide precursor solution in the periphery of substrate can completely be thrown away so as to avoid its adhesion onto the backside of substrate. But, if the viscosity of the polyimide precursor solution is low, a series of Steps 1, 2 and 3, that is, the spin coating treatment of the polyimide film must be repeated in order to obtain a film of a desired thickness. So, this method cannot eliminate the drawback of low throughput, either.